1. Field of the Invention
The invention relates to a steam generator with integrated dust separator, the steam generator consisting of a combustion chamber, a radiation part and a convection part and having at least two vertical gas passes following the combustion chamber, each with 180.degree. gas deflection.
2. Discussion of Background
At the present time, the boilers used for garbage incineration plants (GIP) in Central European countries are preferably those in which the gas flowing out of the combustion chamber flows via a first empty pass with downward flow into a second empty pass with upward flow and subsequently into a horizontal bunching pass. In addition to these horizontal steam generators, vertical steam generators for garbage incineration plants are also known, these usually being of the 3-pass or 4-pass type (K. J. Thome-Kozmiensky: Thermische Abfallbehandlung [Thermal Waste Treatment]. EF Verlag fur Energie und Umwelttechnik GmbH, 1994, Pages 390 to 402). Grate furnaces with a connected multipass boiler are also often employed in the incineration of solids, such as wood and biomass. In the empty passes, inter alia, fly ash is separated and discharged. Fly ash is a problem material, particularly in garbage incineration, since fly ash is highly contaminated and has to be disposed off as special garbage.
The empty passes of steam generators have hitherto been designed predominantly as rectangular ducts, their longitudinal sections being largely identical.
Whilst importance has hitherto only been attached to an operationally reliable discharge of the dust which falls from the walls by being cleaned off or by flaking off, the quantity of dust at the end of the boiler has in the meantime become a quality feature for incineration and the boiler and has to be guaranteed to the customer.
In the furnaces described above, a relatively large amount of dust (2-5 g/Nm.sup.3) is carried along from the combustion chamber. This high dust fraction is undesirable since it causes the following problems:
The dust settles on the heating surfaces, so that these have to be overdimensioned correspondingly. Moreover, they have to be built in such a way that they can be cleaned by tapping or soot blasting. PA1 In the gas scrubbing plant downstream of the boiler, costly electrostatic filters and washers have to be used for dedusting. PA1 In the horizontal part of the boiler, the precipitated dust is conducive to the catalytic formation of dioxin. This reaction (Denovo synthesis) takes place when gas containing oxygen and chlorine fractions comes into contact, at a temperature of between 250 and 500.degree. C. and with a dwell time of several seconds, with a catalyst (in the present case, the dust acts as a catalyst). Flue gas and dust, which are virtually free of dioxin after incineration, become dioxin-containing again below 500.degree. C. as a result of this Denovo synthesis. PA1 The dumping of dioxin-containing dust is a costly environmental problem. PA1 The scrubbing of dioxin-containing waste gases is highly complicated.
Recent tenders demand a dust content of crude gases of 1-2 g/Nm.sup.3 which can no longer be adhered to by means of firing measures alone. For a "high dust-SCR (Selective Catalytic Reduction)" plant (NOx catalysts are arranged upstream of flue gas scrubbing and upstream of the electrostatic filter), even a dust content of &lt;1 g/Nm.sup.3 is required, since, because of this arrangement, the flue gas denitrogenation reactor is loaded with dust and has only a reduced service life.
DE 28 05 671 A1 and EP 0 177 667 A1 disclose devices for improving the flow deflection of flue gases in a multipass boiler, in which devices guide plates or n-bladed blade cascades are arranged in the region of the deflection of the vertical pass. These are designed in such a way that, as far as possible, no breakaways (less pressure loss, fewer dust deposits) occur. As compared with steam generators having vertical passes without such guide plates or guide cascades in the deflection region, these solutions regarding the gas flows, although constituting an improvement, are nevertheless completely unsuitable in terms of effective dust separation.
EP 0 748 647 A1 discloses a method and an apparatus for reducing the dust content in steam generators having at least two vertical passes of the type described above. The method is defined in that, in the region of gas deflection from the vertical pass with downward flow to the vertical pass with upward flow, dust separation at this point is improved by the provision of flow-calmed zones and the dust is discharged directly from this region. The corresponding apparatus is defined in that, in these regions, the front wall and/or rear wall of the steam generator is of double-walled design, the flow-calmed zones, in which the dust is separated effectively, being located between the two walls. In a design variant, the partition between the vertical passes is bent in its lower region in the direction of the vertical pass with downward flow, so that the cross-sectional area of the gas duct is reduced in this region.
The disadvantage of such a solution, however, is that the additional fittings (second inner wall with baffle plates) are at risk of becoming clogged.